These machines make use of a technique which is undergoing constant change. Especially in the automobile industry, this machining technique is being more and more widely adopted in factories for the large-scale production of parts. Its main advantages lie in highly accurate performance, good finish or in other words a good state of surface, and a high production rate. Although broaches are specially designed for each workpiece and their cost is therefore very high, the process of machining by broaching proves to be highly economical when applied to large-scale production.
Broaches and machining jigs are constantly becoming more modern, more complex and more accurate. Broaching machines follow this evolution only with difficulty. Tests have shown that the state of surface of machined parts varies with the cutting speed; above approximately 25 m/min, it has been found that the state of surface is as good as at low speed, especially in the case of a substantial depth of pass. In point of fact, broaching machines of the type in use at the present time are designed to operate at cutting speeds which hardly exceed 20 m/min, that is, within an unsatisfactory speed range.
In these machines the motion of the tool-carrier slide-block is often controlled by a hydraulic jack. This solution is well suited to low slide-block speeds but in the vicinity of 25 m/min and above this rate, the problems of rigidity of the spindle, of output of the hydraulic power unit, of pressure drops, overheating of oil, pressure surges and so forth become practically insoluble in the present state of the technique.
An attempt has also been made to drive broaching machines by means of an electric d.c. motor but this solution is also attended by drawbacks. By reason of the fact that the motor produces accelerations, slowing-down, stopping and reversing of the direction of the slide-block, said motor must be controlled by a speed-changing device which is both powerful and reliable and therefore relatively costly. The weight of the moving system constituted by the slide-block, the broach, the rotor of the motor and the elements of the transmission system placed between the rotor and the slide-block is substantial, especially in machines of large size. In consequence, the positive or negative accelerations which have to be imparted to the slide-block result in substantial current surges and overheating of the motor which have an adverse effect on its length of life.
These disadvantages become of greater importance as the cutting speeds to be attained become higher and increase the variations in kinetic energy of the moving system. Moreover, whereas it is only necessary in the case of horizontal machines to take into account the accelerations of the moving system, it also proves necessary in vertical machines to overcome the acceleration of gravity at the time of upward motion of the slide-block.